Taken from the manual for Dell Optiplex GX270
Booting From the Operating System CD
1. Save and close any open files and exit any open programs.
2. Insert the Operating System CD. If any program starts automatically, exit the program before proceeding.
3. Shut down the computer through the Start menu and restart the computer.
4. Press <F12> immediately after the DELL™ logo appears.
If the operating system logo appears, wait until you see the Windows desktop, and then shut down the computer and try again.
5. Press the arrow keys to select CD-ROM, and then press <Enter>.
6. When the Press any key to boot from CD message appears, press any key.
Entering System Setup
1. Turn on or restart your computer.
2. When Press <F2> to Enter Setup appears in the upper-right corner of the screen, press <F2> immediately.
If you wait too long and the Microsoft® Windows® logo appears, continue to wait until you see the Windows desktop. Then shut down your computer
through the Start menu and try again
Changing Boot Sequence for Future Boots
1. Enter system setup.
2. Use the arrow keys to highlight the Boot Sequence menu option and press <Enter> to access the pop-up menu.
3. Press the up- and down-arrow keys to move through the list of devices.
4. Press the spacebar to enable or disable a device (enabled devices have a checkmark).
5. Press the plus (+) or minus (–) keys to move a selected device up or down the list.
Hope that helps